1. Field of the Invention
The present invention relates generally to a ceramic green sheet, more particularly, to a ceramic green sheet which is used for providing an electrode protective layer of an electrochemical element adapted to detect or determine the concentration of a given component of a subject gas to be measured. The invention is also concerned with an electrochemical element which has an electrode protective layer prepared from such a ceramic green sheet, and a method of producing such an electrochemical element.
2. Discussion of the Prior Art
Porous ceramic structures or layers having a multiplicity of mutually connected pores have been widely used for various purposes. For example, a porous ceramic layer is used as a gas-permeable protective layer which covers an electrode of an oxygen sensing element or other electrochemical element for determining the concentration of a given component of a subject gas. This porous protective ceramic layer is used for protecting the electrode which is directly exposed to the subject gas, e.g., an exhaust emission produced by internal combustion of an air-fuel mixture. Namely, the porous protective layer serves to protect the electrode against adverse influences of corrosive substances, such as lead, phosphorus, silicon and sulfur. When the electrode is exposed to these corrosive substances, the sensing element tends to suffer from a change in the control point, reduction in the output, and deterioration of the operating response. The porous protective layer also serves to prevent volatilization of the electrode components at an elevated temperature, thereby assuring improved durability of the sensing element. Where cracks occur in the protective layer during use of the sensing element, therefore, the electrode is directly exposed to a corrosive gas having a high temperature, resulting in corrosion and volatilization of the exposed electrode. In this case, the sensing element substantially loses its sensing capability.
The porous ceramic layer used as an electrode protective layer is formed by plasma-spraying a suitable ceramic material such as spinel so as to cover the electrode, and/or a substrate such as a solid electrolyte body on which the electrode is formed. Alternatively, a green sheet of a suitable ceramic material is laminated on a solid electrolyte body or other substrate, so as to cover the electrode formed on the substrate, and the green sheet is fired into the porous electrode protective layer integral with the substrate. Generally, the porous ceramic layer prepared from a ceramic green sheet is given a suitable porosity, in one of the following manners: (1) The green sheet is fired such that the sintering of the ceramic powder is insufficient; (2) The green sheet contains a relatively large amount of binder; and (3) The green sheet contains an organic additive or additives other than the binder, which additives are burned out during firing of the green sheet.
The porous ceramic layer formed by the plasma spraying technique has low mechanical strength, and tends to suffer from cracking or flake-off due to a thermal shock or stress. Further, it is generally difficult to practice the plasma-spraying technique, when the desired thickness of the porous ceramic layer exceeds 100 .mu.m.
The porous ceramic layer formed by the green sheet lamination method has a multiplicity of pores which provide relatively short fluid passages or channels (gas-permeation paths) through which a gaseous fluid flows, for example. In this case, the gaseous fluid flowing through the passages immediately reaches the surface of the substrate on which the porous layer is formed. Accordingly, when the porous ceramic layer is used as an electrode protective layer of an electrochemical element, such as an oxygen sensing element, the protective layer is not able to sufficiently remove corrosive substances contained in a subject gas. Consequently, the sensing element thus obtained tends to suffer from a change in the control point, reduction in the output, and deterioration of the operating response, and is therefore unsatisfactory in its durability.